Enterprise data systems, especially for large enterprises, typically include multiple data sources that may be based on different data schemas like                relational databases,        XML documents, XML schema, XML document type definitions (DTD),        web services,        Adabas files        SGML texts,        Interface Description Language (IDL),        JAVA interface,        Resource Description Framework (RDF),        Unified Modeling Language (UML),        Ontology Web Language (OWL),        Object role modelling,        Topic maps,        etc.        
As a result of several IT generations and/or mergers and acquisitions, several databases with different schemas may contain information on different areas of the business. For example, enterprise employment data may be stored in a relational database conforming to a first relational database schema and enterprise accounting data may be stored in a relational databases conforming to a second relational database schema. Often these various data sources may overlap, and it is difficult for a user to query across the enterprise data sources.
There is, thus, a need for tools for data integration or information integration and a need for a unified querying tool that enables a user to query data across data sources conforming to different data schemas. The need for a unified querying tool also arises when a single question crosses multiple data sources.